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HY436: Mobile Computing and Wireless Networks
IEEE802.11 Lecture 5: October 18, 2004
Prof. Maria Papadopouli Assistant Professor
Department of Computer Science University of North Carolina at Chapel Hill
Mechanics Log (in English) Clear problem description for each assignment Clear description of the format for the
generated data Well-documented code Another project: Enhancements, measurements and evaluation of
the 2wear-related applications http://2wear.ics.forth.gr
Coordination functions for channel access Distributed Coordination function
Contention-based access DIFS ms sensing channel 4-way handshaking protocol for data
transmissions Backoff process
Point Coordination function Contention-free access
Using the NAV for virtual carrier sensing
Every host that receives the NAV differs the access, even if it is configured to be in a different network
Inter-frame spacing
Create different priority levels fro different types of traffic
PCF (contention-free) accessPreempt any contention-based traffic
Minimum medium idle time for contention-based services
Fragmentation burst
A day in the life of an 802.11 Client
Frame Format Power Management Security Protocols
A Network of Socialites
Our 802.11 station would like to Join the community (i.e., a network) Chat for a while (send and receive
data) Take a nap (rest, then wake up) Take a walk (roam to a new area) Leave the network
Background A frame is a chunk of data without control data A MAC Address MAC Address is a 48-bit
permanent ID number on each station in IEEE 802 protocols (e.g.: “00:30:65:01:ed:7e”)
An internet interconnects link-level interconnects link-level physical networks; data transits in physical networks; data transits in packets
Two Modes of Operation of the 802.11 Device
Infrastructure: A special STA, the Access Point (AP), mediates all traffic mediates all traffic
Independent: Stations speak directly to one another
Steps to Join a Network
1. Discover available networks (aka BSSs)2. Select a BSS 3. Authenticate with the BSS4. Associate
Discovering Networks
Each AP broadcasts beacons announcing itself
Beacon includes: AP’s MAC address AP’s clock Beacon interval (100ms typical) Network Name (SSID); eg “UNC-1”
Associations
Exclusive: A device can be associated with only one
AP Client-initiated:The device initiates the association process AP may choose to grant or deny access
based on the content of the association request
Reasons to Deny Access
Memory Traffic load
Networks of Arbitrarily Large size
• Chain BSSs together with a backbone network• Several APs in a single area may be connected to a single hub or switch or they can use virtual LAN if the link=layer connection
APs act as bridges
Backbone network is a layer 2 (link layer) connection
APs are configured to be part of the ESS
Basic Service Set: the networkaround one AP
Inter-Access Point Communication
If a client is associated with one AP, all the other APs in the ESS need to learn about that client
If a client associated with an AP sends a frame to a station associated with a different AP, the bridging engine inside the first AP must send the frame over the backbone Ethernet to the second AP so it can be delivered to its ultimate destination
No standardized method for communicationMajor project in the IEEE802.11 working group the
standardization of the IAPP
Infrastructure Mode: Joining a network 1. Discovering Networks (active)
1. Instead of waiting for beacon, clients can send a probe request which includes
STA MAC address STA’s supported data rates May specify a SSID to restrict search2. AP replies with proble response frame
Infrastructure Mode: Joining a network 2. Choosing a Network
The user selects from available networks; common criteria: User choiceStrongest signalMost-recently used
OS Driver indicates this selection to the STA
Infrastructure Mode: Joining a network 3. Authentication
Open-system ‘authentication’; no password required
Often combined with MAC-address filtering
Infrastructure Mode: Joining a network 3. Authentication
Shared-key ‘ authentication’ called “Wired Equivalency Protection”, WEP
What’s wrong with WEP Apparent design mistakes make a WEP key
recoverable Result 40-bit key on a busy network recoverable in five
hours 128-bit key in 15 hours Improving on WEP without changing STA:
Have lots of keys (e.g., one per user) simultaneously; time to crack increases with number of keys
Change keys frequently
What’s wrong with WEP
802.1x supports “logging in” to a network, and automatic distribution of WEP data
802.11i uses 802.1x and adds short-lived keys
Infrastructure Mode: Joining a network 4. Association
Station requests association with one AP Request includes includes
STA MAC address, AP MAC address, SSID (Network name), Supported data rates, Listen Interval (described later)
We have now joined the network …
Next: sending data
Sending a frame
1. Request to Send – Clear to send Used to reserve the full coverage areas of both
sender and receiver1. Send frame2. Get acknowledgement
Infrastructure mode: Sending Data 1. RTS/CTS
RTS announces the intent to send a pkt; it includes: Sender’s MAC address Receiver’s MAC address Duration of reservation (ms)
CTS inidcates that medium is available; includes: Receiver’s MAC address Duration of reservation remaining (ms)
Infrastructure mode: Sending Data 2. Transmit frame
Normal ethernet frame has two addresses: sender and receiver
802.11 data frame has four possible addresses: Sender (SA) originated the data Destination (DA): should ultimately receive the data Receiver (RA): receives the transmission from the sender Transmitter (TA) transmits the frame
Data frame includes also Duration remaining in fragment burst More-fragments ? Indicator Data
Frame Control Field
AP indicates that there are more data available and is addressed to a dozing station
Indicates if the device is sleeping
Data sent …
Next: Take a nap
Power Savings: Basic Principle
Whenever a wireless node has noting to send or receive it should fall asleep: turn off the MAC processor, the base-band processor, and RF amplifier to save energy
Easy in an infrastructure wireless network APs responsible for timing synchronization
(through beacons)
Infrastructure mode: Saving Power
1. STA indicates power management mode is on to AP and waking interval
2. STA goes to sleep (turns off radio)3. STA wakes later; Listens for traffic conditions (e.g., first 10ms
of the beacon interval)4. STA may request buffered frames5. AP sends buffered frames
Steps 2-5 repeat
1. STA indicates Most frames include power-management (PM) bit;
PM=1 means STA is sleeping STA also indicates Listen Interval; length of its naps (in
beacon intervals)
Tradeoffs:
Higher listen interval requires more AP memory for buffering Interactivity issues APs may use this feature to estimate the resources that will be required and may refuse resource-intensive associations
Infrastructure Mode2. Check for waiting traffic
Station wakes to listen for a beacon, which includes the Traffic-Indication Map (TIM)
TIM is 2,007-bit-long map; TIM[j]=1 means that station with
Associated ID=j has traffic buffered
Infrastructure Mode3. Get buffered traffic
Station sends Power-Saving-Poll to indicate that it is awake and listening
AP sends buffered packets Station stays awake until it has retrieved
all buffered packets
Infrastructure Mode: RoamingRe-association
When a station leaves one BSS and enters another BSS, it can re-associate with its new AP
Re-association request is like association plus: Previous AP MAC address Old association id
New AP can contact old AP to get buffered frames
Infrastructure mode: Leaving the network
If a station is inactive, AP may disassociate it automatically; 30 seconds is typical
Station may indicate its de-association politely
Polling-based (centralized) Point Coordination Function (PCF)
TDMA scheme Point-coordinator cyclically polls all stations which are
assigned to the network and added to the PC polling table
Assign a time slot to them in which they are exclusively allowed to send data
Drawbacks:Higher bandwidth waste under normal load
Correction (for reducing overhead for polling idle stations)Embedded Round Robin: dynamic classification of stations as busy or clear